Equipment for biological water treatment, in particular for denitrification of raw water to produce potable water

ABSTRACT

Known types of equipment for the biological denitrification of potable water require (back) washing of the reactors from time to time. These denitrification processes proceed therefore in a discontinuous manner. The invention proposes a continuously operating denitrification process with a simple equipment arrangement. This is achieved by sumberged drum reactors (12, 14) which rotate in a first, anoxic biological stage (10) and in a second, aerobic biological stage (13). The immersion bodies (immersion body segments 31) contained in these reactors are, due to the rotation, continuously washed during the treatment of the potable water. Moreover, the sumberged drum reactors (12, 14) of the two biological stages (10, 13) are arranged according to the invention on a common drive shaft (26) with a drive element (drive motor 27).

This is a divisional of application Ser. No. 07/121,835, filed Nov. 17,1987, U.S. Pat. No. 4,935,130, which is a continuation of applicationSer. No. 06/800,939, filed Nov. 22, 1985, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for biological water treatment, inparticular for denitrification of raw water to produce potable water.The invention also relates to equipment suitable for biologicaldenitrification, in particular of potable water.

2. Description of the Related Art

The raw water used for the production of potable water, for exampleground water, is increasingly subject to environmental pollution. Aboveall, nitrates from fertilizers, manure or the like pass into the groundwater. The potable water produced from the latter must therefore befreed from nitrate (denitrified) before consumption or at least to suchan extent that it meets the statutory requirements. It can also becomenecessary to denitrify treated industrial water.

Multi-stage denitrification is known, in which the nitrate-containingraw water is first passed, with the addition of a reducing agent, forexample ethanol, glucose or the like, into an anoxic reactor packed withcarrier materials. As a result of the added reducing agent, bacteriawhich reduce the nitrate to molecular gaseous nitrogen are formed on thecarrier material. At the same time, the carbon-containing reducingagents provide an organic carbon supply, namely as an energy carrier,for the bacteria in the reactor. The potable water pretreated to thisextent then requires an aerobic, biologically active filtration. In sucha filter, the excess substances added as reducing agents and biomass areto be removed from the potable water, with addition of oxygen.

A disadvantage of this known process is the discontinuous course of thedenitrification. This results from the fact that the reactors must bewashed at regular intervals in order to remove the excess biomassproduced. Moreover, the quality of the treatment in the first processstage depends on the age of the biomass in the anoxic reactor. Anexpensive combination of processes for final purification of thedenitrified water is therefore necessary in every case. For this reason,the known biological denitrification process requires careful processsupervision and intensive servicing of the unit. With known staticcarrier material packing, there is a risk, in the event of uneven flowthrough it, of the biomass caking, as a result of which undesirednitrite can be formed.

This known process also has disadvantages in respect of equipment. Infact, in order to avoid the complete close-down during the frequentlynecessary washing of the reactors, several reactors should be provided,of which alternately one reactor is always in operation, while the otherreactor is being back-washed.

SUMMARY OF THE INVENTION

It is therefore the object of the invention to provide a process whichis easy to control, in particular a continuous process, and inexpensiveequipment of simple structure for the denitrification, in particular ofpotable water.

To achieve this object, the process according to the invention has theresult that, in the second stage with supply of oxygen, the aerobicmicroorganisms degrade the secondary matter from the first stage, namelyexcess reducing agents and a part of the biomass discharged from theanoxic reactor. This has the advantage that, in the process according tothe invention, a simple mechanical filter can be used as the downstreamfilter, that is to say biological filter, such as is necessary in thedenitrification process of the state of the art, which is difficult tocontrol on line, can be omitted. Since the secondary matter isbiologically degraded, the residual biomass which may still be presentin the potable water after the mechanical filtration is very largelyharmless.

According to the process, it is also proposed to use rotary submergeddrum reactors in both water treatment stages, namely the first anoxicbiological stage and the second aerobic biological stage. Due to thecontinuous rotary movement of the reactors in the vessel with thepotable water to be treated, excess biomass or reducing agents arecontinuously washed out of the immersion bodies located in the submergeddrum reactors. Only a thin, but biologically active layer of bacteriathus remains on the immersion body surfaces. The back-washes required inthe state of the art are accordingly unnecessary. Therefore, thedenitrification process according to the invention allows virtuallyuninterrupted operation of the unit. Blockages of the immersion bodiesare prevented, since they continuously move in the potable water to betreated.

At the same time, on the one hand, there is even continuous washingthrough the immersion bodies and, on the other hand, controlled flow ofthe water to be treated through the vessels of the individual treatmentstages takes place, so that optimum nutrient absorption and optimum gasexchange are obtained.

The equipment according to the invention for achieving this objectconsists of conventional components. The submerged drum reactors caneasily be installed into and removed from the appropriate vessels andare easy to control.

According to a further proposal of the invention, the submerged drumreactors in the individual vessels are of approximately identical designand are mounted on a common drive shaft. Since identical submerged drumreactors are used, these can be economically mass-produced. The mountingon a common drive shaft provides a compact installation which requiresonly one drive. Depending on the capacity of the unit, one or severalsubmerged drum reactors can be arranged in series within one vessel,that is to say within one treatment stage.

Advantageously, the vessels, receiving the submerged drum reactors, forthe individual treatment stages should be arranged, for space reasons,either immediately side by side or at a small spacing.

According to the invention, the submerged drum reactors are composed ofa three-dimensional support structure and the immersion body arrangedtherein. In an advantageous embodiment of the invention, the immersionbodies in turn consist of a plurality of immersion body segments. Inthis way, the submerged drum reactors can be assembled from smallercomponents (which are easy to handle). In the event of faults occurringin the submerged drum reactor, individual segments can be replaced.

The support structure is composed of profile bars which run radially tothe longitudinal centre axes of the submerged drum reactors and arearranged in such a way that they guide the individual immersion bodysegments along their radially directed edges. To secure the individualimmersion body segments against dropping out of the drum reactors,clamping rings are used which surround the outer periphery of the drumreactors and, formed either integrally or likewise as segments, areconnected to the free ends of the profile bars of the support structure.

In the first, anoxic biological stage, operating with exclusion ofoxygen, the vessel receiving the submerged drum reactor may be designedto be gas-tight. In this stage, about half of the submerged drum reactorcan be immersed into the water to be treated. As a result, the bacteriagrowing on the immersion body can start nitrate respiration after ashort initial phase, that is to say reduce the nitrate ion to gaseousnitrogen by utilizing the three oxygen atoms bonded in the ion. Sinceair is excluded from the vessel, the brief emergence of a part of thesubmerged drum reactor from the water does not adversely affect theactivity of the bacteria film. Preferably, the denitrification in thefirst, biological stage can be carried out in an open vessel, but with asubmerged drum reactor which is completely immersed into the water to betreated. In this case, the continuing exclusion of oxygen feed from thebiological film, as necessary for the nitrate reduction, is the resultof the fact that the immersion body segments are continuously immersedin the water to be treated. In other words, the submerged drum reactoror reactors in the first (anoxic) biological stage is or are completelyimmersed into the (potable) water to be treated, in order to isolate thesubmerged drum reactor from oxygen.

In the second, aerobic biological stage, the treatment takes place in anopen vessel with oxygen supply. In this treatment stage, about half ofthe submerged drum reactor is immersed into the denitrified water. Dueto the rotation of the submerged drum reactor, the water is continuouslyaerated for the formation of an aerobic bacteria film on the immersionbody of this treatment stage. The aerobic bacteria thus consume theremainder of the excess reducing agent metered into the first treatmentstage and a part of the biomass discharged during the denitrificationfrom the first submerged drum reactor.

According to the invention, the vessels of the two treatment stages aremutually connected by an overflow. The latter is arranged in such a waythat a water level which is higher than that in the second treatmentstage is automatically established in the vessel of the first treatmentstage, in order to ensure the required depths of immersion of thesubmerged drum reactors which effect different treatments. As a result,expensive control systems for adjusting the required levels in theindividual treatment vessels can be omitted.

Finally, the invention proposes to place the overflow, a water feed intothe first treatment vessel and a water discharge from the secondtreatment vessel in the vicinity of the corners of the vessels,especially in a zig-zag form, so that the water feed and water dischargeare approximately diagonally opposite in each vessel. In this way, aformation of dead zones and short-circuit flows while water flowsthrough the individual vessels is avoided. As a result, there isintensive flow around the biological film on the immersion bodies. Atthe same time, the formation of dead zones with stagnant water in thevessels is avoided, in favour of likewise intensive treatment of thepotable water.

Further features of the invention relate to the constructional design ofthe support structure and of the immersion body segments of the drumreactors.

BRIEF DESCRIPTION OF THE DRAWINGS

An illustrative embodiment of the invention is explained in more detailbelow by reference to the drawings in which:

FIG. 1 shows a diagrammatic side view of the equipment,

FIG. 2 shows a diagrammatic plan of the equipment according to FIG. 1,

FIG. 3 shows a perspective overall view of a submerged drum reactor,

FIG. 4 shows a cross-section, partially shown enlarged, through thesubmerged drum reactor according to FIG. 3,

FIG. 5 shows a partial side view of the submerged drum reactor accordingto FIG. 4,

FIG. 6 shows a perspective illustration of an immersion body segment,

FIG. 7 shows a side view of a section of an open-mesh pipe, and

FIG. 8 shows a cross-section VII--VII through the open-mesh pipeaccording to FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present illustrative embodiment relates to compact equipment for thebiological denitrification of potable water.

The equipment consists of a (first) anoxic biological stage 10 with avessel 11, in which two submerged drum reactors 12 are arranged side byside, of a (second) aerobic biological stage 13 with a vessel 15containing a submerged drum reactor 14, and of a mechanical filter unit16 of a design type known per se.

The vessel 11 of the first biological stage 10 is open and anoxicconditions are ensured by a complete submersion of the drum reactors inwater. In cross-section (not shown), the lower half of the vessel 11approximately matches the curvature of the submerged drum reactors 12,that is to say, in the lower half, the wall of the vessel 11 extends ata spacing from and approximately parallel to the submerged drum reactors12.

In the present illustrative embodiment, the water level 18 in the vessel11 of the first biological stage 10 is above the submerged drum reactors12. An overflow pipe elbow 20 connecting the vessels 11 and 15 servesfor maintaining the envisaged water level 18 in the vessel 11. For thispurpose, a horizontal section, ending in the vessel 11, of the overflowpipe elbow 20 is arranged at a height above the water level 18. Avertical pipe section, leading into the vessel 15, of the overflow pipeelbow 20 ends below the water level 21 in the vessel 15 of the secondbiological stage 13.

The water to be treated, namely the ground water provided with reducingagents, passes through a feed pipe elbow 22 into the vessel 11 of thefirst biological stage 10. In this illustrative embodiment, thehorizontal pipe section of the feed pipe elbow 22 enters an upper regionof the vessel 11 and its (long) vertical pipe section protrudes deepinto the water which is to be denitrified in the first biological stage10. In the present illustrative embodiment, the feed pipe elbow 22enters the water from above, at about one third of the height of thewater level 18. The overflow pipe elbow 20 and the feed pipe elbow 22are allocated to approximately diagonally opposite corner regions of thevessel 11 of the first (anoxic) biological stage 10.

The upper side 23 of the vessel 15, which in this case has only onesubmerged drum reactor 14, of the second (aerobic) biological stage 13is not closed. Thus, in contrast to the vessel 11, aeration of the waterto be treated is possible in this vessel 15. The water level 21 in thevessel 15 is slightly above the longitudinal centre axis 19 of the drumreactor 14. The water level 21 of the second biological stage 13 is thusbelow the water level 18 of the first biological stage 10. The envisagedwater level 21 in the level 15 is maintained--as in the vessel 11--bymeans of a discharge pipe 24 arranged at an appropriate height. Thedenitrified potable water passes through this discharge pipe into themechanical filter unit 16. Commercially available, mechanical filterunits can be used for this purpose, provided that they do not conflictwith continuous operation of the biological stages 10 and 13.

In the present illustrative embodiment, the vessel 11 and 15 haveidentical--approximately trough-shaped--cross-section. The vessels 11and 15 are arranged one immediately behind the other, with a commoncentral partition 25. The discharge pipe 24 in turn is arranged in thecorner region of the vessel 15, diagonally opposite the overflow pipeelbow 20. The feed pipe elbow 22 for the vessel 11 and the dischargepipe 24 in the vessel 15 are therefore approximately opposite oneanother (FIG. 2). With respect to height, the discharge pipe 24 and theoverflow pipe elbow 20 are offset corresponding to the different waterlevels 18 and 21 in the vessels 11 and 15 respectively, since thedischarge pipe 24 is in fact arranged lower down.

The two submerged drum reactors 12 in the vessel 11 and the submergeddrum reactor 14 in the vessel 15 are mounted on a common, continuousdrive shaft 26 (FIGS. 1 and 2). The shaft extends along the longitudinalcentre axes 19 of the submerged drum reactors 12 and 14, so that thelatter are located in series at the same height.

All the submerged drum reactors 12, 14 are driven by a common electricmotor 27 which is arranged outside the vessels 11 and 15 (FIGS. 1, 2,4). To reduce the motor speed to a relatively low drive speed of thedrum reactors 12, 14, a gearbox 28 (FIGS. 1, 2) between the drive shaft26 and the drive motor 27 or alternatively an open cog wheel drive,consisting of a relatively large cog wheel 28 allocated to the frontsubmerged drum reactor 12 in the vessel 11 and of a small pinion 29associated with the drive motor 27, can be provided (FIG. 4).

In the present illustrative embodiment, the three submerged drumreactors 12, 14 are of the same design. Each submerged drum reactor 12,14 consists of a three-dimensional cylindrical support structure 30which receives the immersion body consisting in the present illustrativeembodiment of a total of ten immersion body segments 31 (FIG. 3).

The support structure 30 consists of radially directed, T-shaped profilebars 32 which are located in two mutually spaced, upright (end) planesof the submerged drum reactors 12, 14. The ends, pointing to the centreof the submerged drum reactors 12, 14, of the profile bars 32 are joinedto the drive shaft 26 by collars 33 fitted thereto (FIG. 4). At the endfaces of the submerged drum reactors 12, 14, the profile bars 32 arebraced to one another by transverse struts 34 (FIG. 5). In the radiallydirected plane, transversely thereto, between two immersion bodysegments 31, the profile bars 32 mutually adjacent in pairs are alsostrutted, namely by a bracing 35 which extends in the shape of an X tothe ends of the profile bars 32 (FIG. 4).

The dimensions of the support structure 30 and the arrangement of theprofile bars 32 are such that the total of ten immersion body segments31, provided in this illustrative embodiment, can be inserted from theoutside into the support structure 30. For doing this, the immersionbody segments 31 are guided in the corner regions of the radiallydirected longitudinal edges by the T-shaped profile of the profile bars32.

In the region of the hub of the support structure 30, cross struts 36extending parallel to the drive shaft 26 at a distance are provided.These limit the depth of insertion of the immersion body segments 31into the support structure 30.

On the outer periphery, the support structure 30 has two clamping rings37, which are each allocated to an (end) plane formed by the profilebars 32. In the present illustrative embodiment, the clamping rings 37are each assembled from ten ring segments 38. These ring segments eachconnect the outer free ends of two profile bars 32 lying in one plane.For this purpose, each profile bar 32 has, on its free outer end, abracket 39 to which two opposite ring segments 38 are bolted. Thearrangement of the ring segments 38 of the support structure 30 is suchthat these segments cover the edge regions of the outer curved surfaceof the immersion body segments 31 from the outside of the submerged drumreactors 12, 14, in order to secure the immersion body segments 31 inthe submerged drum reactors 12, 14.

Each immersion body segment 31 consists of a multiplicity of open-meshpipes 40 which run parallel to the longitudinal centre axis 19 of thesubmerged drum reactors 12, 14 (FIG. 3, FIG. 6). The individualopen-mesh pipes 40 packed together to give an immersion body segment 31are mutually joined by welding of the opposite end faces.

The diameters of the individual open-mesh pipes 40 of an immersion bodysegment 31 can differ. For example, open-mesh pipes 40 of largerdiameter can be arranged towards the interior of the submerged drumreactor 12, 14, whereas open-mesh pipes 40 of smaller diameter are usedon the outside (FIG. 6). Alternatively, it is also conceivable toprovide smaller open-mesh pipes 40 in the interior of the submerged drumreactor 12, 14 than on the outside. This gives a geometrical surfacearea of approximately the same size in all the regions of the submergeddrum reactor 12, 14. The diameters of the open-mesh pipes 40 can be 10mm-70 mm. This can give a geometrical surface area per segment volume ofbetween 100 m² /m⁺³ and 400 m² /m⁺³.

The open-mesh pipes 40 consist of longitudinal fibers 42 crossingcircular fibers 41 of approximately the same cross-sectional dimensions(FIG. 7). Between the longitudinal fibers 42, the open-mesh pipes 40have in each case a longitudinal web 43. The latter has a pointed,triangular cross-section with radially inward-directed spikes 44protruding into the open-mesh pipe 40 (FIG. 8).

The equipment of the illustrative embodiment described operate asfollows:

The ground water to be treated is passed via the feed pipe elbow 22 withaddition of reducing agents into the vessel 11 of the first (anoxic)biological stage 10. In the latter, the bacteria forming on theopen-mesh pipes 40 reduce the nitrate content in the ground water togaseous nitrogen. The potable water denitrified in this way then passesthrough the overflow pipe elbow 20 into the vessel 15 of the secondbiological stage 13. The excess reducing agents and the bacteria, washedout of the first biological stage 10, in the aerated water arebiodegraded here. Finally, water treated in this way passes via thedischarge pipe 24 from the second biological stage 13 into themechanical filter unit 16. The latter filters the excess biomass, purelymechanically, out of the potable water. A final biological treatment inthe mechanical filter unit 16 is no longer necessary.

Due to the common drive of the submerged drum reactors 12, 14 from thedrive shaft 26, the submerged drum reactors 12, 14 in both biologicalstages 10, 13 are moved continuously at the same speed for even washingthrough the immersion body segments 31.

Alternatively, the equipment according to the invention can be used fordenitrifying effluents. An artificial addition of reducing agents ornutrients for the bacteria in the biological treatment stages can thensometimes be omitted, since in most cases these are already present inthe effluent.

We claim:
 1. Equipment for biological denitrification of potable water,comprising:inlet means (22) for receiving nitrate-containing potablewater including reducing agents; an anoxic biological stage (10)comprising: an open first vessel (11), coupled to said inlet means (22),for receiving the water; and a first drum reactor (12) which iscompletely submerged in the water in said open vessel (11) and which isfixed to a rotatable drive shaft (26) extending horizontally along alongitudinal axis (19) through the centre of rotation of said drumreactor; an aerobic biological stage (13) for aerating the water andcomprising: an open second vessel (15); a second drum reactor (14) whichis only partially submerged and which is also fixed to said drive shaft(26); and discharge pipe means (24) for discharging water from saidsecond open vessel (15); a common central partition (25) separating saidopen vessels; and connecting line means (20) connecting said openvessels in series for feeding the water from the first open vessel tosaid second open vessel.
 2. Equipment according to claim 1,characterized in that the connecting line means comprises an overflowpipe elbow (20), which is arranged with respect to its height in such away in the first vessel (11) that a desired water level (18) of thewater to be treated is established therein.
 3. Equipment according toclaim 2, characterized in that said inlet means comprises a feed pipeelbow (22), for feeding the water to be treated into the vessel (11),arranged in a corner region approximately diagonally opposite theoverflow pipe elbow (20) and ending below the water level (18) in thevessel (11).
 4. Equipment according to claim 3, characterized in thatsaid discharge pipe means (24) for the water from the second vessel (15)is arranged on the side opposite the overflow pipe elbow (20) at such aheight that a desired water level (21) of approximately half the heightof the second drum reactor is established.
 5. Equipment according toclaim 4, characterized in that the feed pipe elbow (22), the overflowpipe elbow (20) and the discharge pipe means (24) are arranged to bedistributed, in a zig-zag shape relative to the plane of the vessels(11, 15), over diagonally opposite corner regions of the vessels.